Printed electronics is an emerging area for additive manufacturing used to create electrical devices on various substrates. The appeal to directly print electronic features is driven largely by the advantages offered to reduce process steps, capital equipment costs and the need for product specific tooling. By addressing these points, Direct Write (DW) technologies offer the ability to produce low volume, high mix electronics at a per piece cost normally associated with large production volumes where economy of scale aids reducing part cost. There is continuous focus on providing the ability to print finer and finer features using the DW technologies.
One leading technology in the DW area is Aerosol Jet (AJ) printing technology wherein aerodynamic focusing is used to print features down to approximately 10 μm. The AJ process begins with the atomization of an ink, producing droplets approximately 1-2 microns in diameter. The atomized droplets are entrained in a gas stream and delivered to a print head. The combined gas streams exit the print head at high velocity through a converging nozzle that compresses the aerosol stream to a small diameter. The high exit velocity of the jet enables a relatively large separation, typically a few millimeters, between the print head and the substrate. Although the AJ technology has been demonstrated in a laboratory environment, several technical issues persist that limit the use of this technology for commercial applications. These issues include:                1. Shuttering—The mechanical shutter used for AJ printing limits its usefulness for conformal printing and has a tendency to eject excess ink collected in the shutter during printing onto the print surface. Shutter response time (approx. 10 ms) limits high speed processing capabilities.        2. Overspray—Both ultrasonic and pneumatic atomization (UA and PA, respectively) methods used for the AJ technology produce a poly dispersed aerosol. Aerodynamic focusing of poly dispersed aerosols leads to overspray on the edges of printed features since smaller droplets focus at a different plane than the larger droplets (an optical analogy is chromatic aberration).        3. Process Reliability—UA and PA generate aerosol with a broad droplet size distribution and the AJ process only uses the droplets at the small end of the distribution curve. Therefore, aerosol usage rate is less than 0.001%. As such, excess energy input to the ink changes ink characteristics over time and affects the output of the printer. Low volatility solvent-based inks can extend print time over high volatility solvent-based inks; however, in both cases total ink utilization is less than 10% before the ink is degraded. Note that there are many good inks that use volatile solvents. To offset this effect, solvent add back has recently been added to all commercial AJ systems. This requires tight temperature control and makes this system more complex.        4. Aerosol Transport—The physical sizes of the UAs and PAs necessitate mounting the atomizer some distance from the print head. Aerosol transport distance to the print head allows settling of the droplets causing clogging or pressure pulses in the transport lines. These pressure pulses affect the print quality.        5. Multiplexing Print Nozzles—AJ has had some success developing multinozzle print heads but, in addition to the above issues, individual nozzle shuttering and uniform aerosol distribution present additional challenges in scaling the AJ technology.        6. Material Output Rate—Aerosol theory shows that diffusion limits the number of droplets that can exist within a given space at any given time and that the number of droplets that can exist is independent of droplet size for the droplet range of interest to AJ printing. This limits the maximum output rate for AJ technology using current UA and PA technology.Therefore, there remains a need for a direct write process that can overcome these problems for printed electronics.        